finalise caching logic

Author: harrysarson

.github/workflows/ci.yml

@@ -16,10 +16,14 @@ jobs:
         with:
           node-version: "${{ matrix.node-version }}"
 
-      - name: Install elm, elm-format and cache the ELM_HOME directory
+      - name: Install elm, elm-test-rs
         uses: mpizenberg/[email protected]
         with:
-          cache-key: elm-home-${{ hashFiles('elm-tooling.json', 'elm.json') }}
+          # The global elm home is _only_ used by elm-tooling to store
+          # installed binaries. For testing we set up our own local elm_home
+          # (and clear it before each run) so we are free to cache elm home
+          # regardless of the packages we use for testing.
+          cache-key: elm-home-${{ hashFiles('elm-tooling.json') }}
 
       - name: Run tests
         run: ./tests/run-tests.sh